How to excess external program and get a value back?

[snippsat]

Use check_output.
It will return output as a whole bytes object so can decode(to string) and split at '/r/n' to get lines.
Example untested:

from subprocess import check_output

command = check_output(["gphoto2", "--get-config", "/main/capturesettings/shutterspeed"])
output = command.decode().strip()
sspeed = output.split('\r\n')[2]

with open('sspeed.txt') as f_out:
    f_out.write(sspeed)